There are a variety of spinal disorders that produce debilitating pain and affect a spinal segment's ability to properly function. The specific location or source of spinal pain is often an affected intervertebral disc or facet joint. A malady in one region of the spine may lead to deterioration and pain in another section of the spine
One common approach to dealing with spinal pain, particularly pain associated with one or more affected intervertebral discs, is spinal fusion or arthrodesis. Spinal fusion is a procedure in which two or more adjacent vertebral bodies are fused together. Although spinal fusion is often effective in eliminating certain types of pain, the procedure has some detrimental effects. The procedure decreases spinal function by limiting the range of motion for the affected joint. Motion is decreased variously in flexion-extension, rotation, and side-to-side bending. The procedure increases the stress on each of the adjacent non-fused motion segments. It accelerates degeneration of those adjacent motion segments.
Prosthetic implantable intervertebral discs are a newer alternative for treating spinal pain. Some prosthetic discs, such as those produced by Spinal Kinetics Inc., Sunnyvale, Calif., preserve the natural biomechanics of the spine by recreating the full range of motion normally allowed by the elastic properties of the natural disc. Other implantable discs accomplish but a limited number of the properties and movement of the natural disc.
Other sources of spinal pain include the facet joints. They may be deformed, arthritic, injured, etc. Such disorders may lead to spinal stenosis, degenerative spondylolisthesis, and isthmic spondylolisthesis, and pinching of the nerves that extend between the affected vertebrae. Current treatments include removal of the facet joints (facetectomy). Such treatment may provide pain relief, but since the facet joints help to support various loads on the spinal column in addition to providing a sliding articulation and mechanism for load transmission, their removal corrupts natural spinal movement. Laminectomy (removal of the lamina, including the spinal arch and the spinous process) may also provide pain relief associated with facet joint disorders; however, the spine is made less stable by such removal.
Various attempts have been made at facet joint replacement. Many such replacements are inadequate since the involved prosthetic facet joints preserve existing bony structures but do not address pathologies that affect the facet joints themselves. Certain facet joint prostheses, such as disclosed in U.S. Pat. No. 6,132,464, are intended to be supported on the lamina or the posterior arch. Since the lamina are very complex and highly variable anatomical structures, it is very difficult to design a prosthesis that provides reproducible positioning against the lamina and to correctly locate the prosthetic facet joints. Prosthetic facet joints such as disclosed in U.S. Pat. No. 6,579,319, require replacement of the natural facet joint and are unlikely to endure the loads and cycling experienced by the vertebra. Such prosthetic facet joints may shift over time. Further, such prosthetic joints do not treat disease or trauma to other structures of a vertebra, such as the lamina, spinous process, or transverse processes.
Dynamic posterior stabilization surgical procedures address spinal pain resulting from more than one disorder, specifically when more than one structure of the spine has been compromised. Many such procedures and structures are intended to provide support to fusion-based implants while maximizing the natural biomechanics of the spine. Such dynamic posterior stabilization systems typically fall into one of two general categories: posterior pedicle screw-based systems and inter spinous process spacers.
Examples of pedicle screw-based systems may be found in U.S. Pat. Nos. 5,015,247, 5,484,437, 5,489,308, 5,609,636, 5,658,337, 5,741,253, 6,080,155, 6,096,038, 6,264,656 and 6,270,498. These types of systems involve the use of screws that are positioned in the vertebral body through pedicles. Certain of these pedicle screw-based systems may be used to augment compromised facet joints, while others require removal of the spinous process or the facet joints for implantation. One such system, the Zimmer Spine Dynesys. employs a cord that is extended between the pedicle screws and a fairly rigid spacer which is passed over the cord and positioned between the screws. Although this system is able to provide load sharing and restoration of disc height, because it is so rigid, it is not effective in preserving the natural motion of the spinal segment into which it is implanted. Other pedicle screw-based systems employ articulating joints between the pedicle screws. Because these types of systems require pedicle screws, implantation of the systems are often more invasive to implant than inter spinous process spacers.
Where the level of disability or pain to the affected spinal motion segments is modest or where the condition, such as an injury, is not chronic, the use of inter spinous process spacers is preferred over pedicle based systems since they require a less invasive implantation approach and less dissection of the surrounding tissue and ligaments. Examples of inter spinous process spacers are disclosed in U.S. Pat. Nos. Re. 36,211, 5,645,599, 6,149,642, 6,500,178, 6,695,842, 6,716,245, and 6,761,720. The spacers are placed between adjacent spinous processes. Spacers produced from harder materials are fixed in place by opposing or squeezing forces on the spacer imposed by the distraction of the affected spinal segments or by use of keels or screws that anchor into the spinous process. Although implanting hard or solid inter spinous process spacers is less invasive than implanting a pedicle screw-based dynamic stabilization system, such procedures still require dissection of muscle tissue and of various spinous ligaments. Additionally, these harder devices promote spinal motion that is less analogous to the natural spinal motion than are the more compliant and flexible inter spinous process spacers. Additionally, delivery of compliant or flexible inter spinous process spacers is less invasive than is the delivery of those that are not compliant or flexible. However, their compliancy makes them more susceptible to displacement or migration over time. To lessen this risk, many such spacers employ straps wrapped around the adjacent spinous processes to situate the spacers. Of course, these straps require dissection of additional tissue and ligament within the adjacent inter spinous process spaces.
None of the cited patents show the structures and procedures described herein.